In a wireless communication system, a base station may transmit downlink data to one or more wireless communication devices (WCDs) that are operating in a wireless coverage area provided by the base station. The wireless coverage area could be, for example, a cell or a sector. The base station may also receive uplink data from one or more WCDs operating in the wireless coverage area.
Some of the downlink data transmitted by the base station may be transmitted in a downlink channel that can be shared among multiple WCDs. For example, the Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) defines a Physical Downlink Shared Channel (PDSCH) as the primary downlink channel for transmitting user data to WCDs. In addition, LTE defines downlink control channels that carry various types of control signaling. The downlink control channels include a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH), and a Physical Hybrid ARQ Indicator Channel (PHICH).
In the LTE approach, downlink resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines 10 millisecond (ms) frames, 1 ms sub-frames and 0.5 ms slots. Thus, each frame has 10 sub-frames, and each sub-frame has 2 slots. In the frequency domain, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), to form one OFDM symbol.
LTE further defines a particular grouping of time-domain and frequency-domain resources as a downlink resource block. In the time domain, each downlink resource block has a duration corresponding to one sub-frame (1 ms). In the frequency domain, each downlink resource block consists of a group of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 1 ms duration of a downlink resource block accommodates 14 OFDM symbols. Depending on the bandwidth of the system, multiple downlink resource blocks can be transmitted in each 1 ms sub-frame. For example, a system with a 5 MHz bandwidth may be able to transmit 25 downlink resource blocks in each 1 ms sub-frame.
The smallest unit of downlink resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a resource block that consists of 12 sub-carriers and 14 OFDM symbols has 168 resource elements. Within a resource block, different resource elements can have different functions. In particular, a certain number of the resource elements (e.g., 8 resource elements) may be reserved for reference signals used for channel estimation. In addition, a certain number of the resource elements (e.g., the resource elements in the first one to four OFDM symbols) may be reserved for control signals in the PCFICH, PDCCH, and PHICH channels. The remaining resource elements in a downlink resource block can be used for user data in the PDSCH channel.
The PDSCH channel can be shared among WCDs by allocating one or more downlink resource blocks to each WCD. Conventionally, the user data (PDSCH channel) in a downlink resource block that is allocated to a particular WCD is for only that particular WCD. Thus, while the PDSCH channel can be shared by multiple WCDs, the user data in a downlink resource block might be for only one WCD.